A pressure accumulator-type (common rail-type) fuel injection device is known which pressure-accumulates fuel, which is pumped by a high-pressure feed pump, with a pressure accumulator (a “common rail”) and injects this fuel from a fuel injection nozzle into a cylinder of an engine with a predetermined timing.
With such a pressure accumulator-type fuel injection device, even if a rotation speed of the engine is at a slow speed, a predetermined fuel injection pressure can be maintained (the fuel injection pressure will not fall), which contributes greatly to improvements in fuel consumption and increases in power output, due to fuel injection by high pressure.
Anyway, it is known that reducing diameter of a nozzle injection aperture in a fuel injection device is effective for the realization of favorable emissions (cleaning of exhaust gases). However, if something that is even smaller than a current injection aperture diameter is employed at the injection pressure of a conventional pressure accumulator-type fuel injection device (a common rail injection system), injection periods at high engine rotation speeds and high load regions become too long, so this is expected to be disadvantageous for increasing power output.
Further, in recent years, there has been a tendency for higher rotation speeds to be anticipated in small-type diesel engines. Here, airflow speed in an engine cylinder increases substantially proportionally to the engine rotation speed. Therefore, with the same injection pressure, spray is more easily flowed at times of high rotation speeds in comparison with times of low rotation speeds, an air utilization rate in the cylinder falls, and smoke (black smoke) is more likely to be exhausted. Accordingly, in order to remedy this, it is desired that the injection pressure should be made even higher. However, a conventional pressure accumulator-type fuel injection device (common rail injection system) as described above is a structure which pressure-accumulates a constant predetermined pressure in the pressure accumulator (for example, in a current common rail injection system, a maximum injection pressure is of the order of 130 MPa). With regard to strength of the device, there is a limit to increases in pressure therebeyond (in other words, it is difficult to make a conventionally increased injection pressure a very high injection pressure).
Meanwhile, a fuel injection device in which a pressure intensification device is further provided at such a pressure accumulator-type fuel injection device has been proposed (for example, the publication of Japanese Patent Application Laid-Open (JP-A) No. 8-21332).
In a fuel injection device disclosed in the above-mentioned publication, a pressure intensification device is provided which further pressurizes pressurized liquid fuel delivered from a pressure accumulator (common rail), by action of a switching valve for piston operation. This pressure intensification device is equipped with a pressure intensification piston formed of a large-bore piston and a small-bore piston, and a plurality of fuel lines which communicate with the switching valve for piston operation. Fuel, which has been delivered from a fuel pressurizing pump, is flowed from the pressure accumulator into the pressure intensification device via the switching valve for piston operation, and is further supplied to a fuel chamber for injection control (an injector control chamber), which is for injection nozzle control, and to an injection nozzle. This is a structure which, when fuel is to be injected, controls switching between low-pressure injection, which sends liquid fuel from the pressure accumulator directly (just as it is) to the injection nozzle for injection, and high-pressure injection, which sends liquid fuel that has been further pressurized at the pressure intensification device to the injection nozzle for injection, by a switching valve for fuel injection control, which is provided at the fuel chamber for injection control. Accordingly, a fuel injection state can be set to be appropriate to driving conditions of the engine.
However, in this fuel injection device, there has been a drawback in that the problem described below occurs.
That is, in the fuel injection device described above, a fuel entrance opening area from the pressure accumulator to a large-bore piston side of the pressure intensifier and a fuel exit opening area of a small-bore piston side of the pressure intensifier, which communicates with the switching valve for piston operation, are fixed structures. Therefore, a time history of fuel pressure when the pressure intensifier is operated is primarily determined by fuel pressure of the pressure accumulator. An example thereof is shown in FIGS. 24A and 24B. As shown in FIG. 24A, if a horizontal axis represents time (seconds), a time history of fuel pressure downstream of the pressure intensifier does not depend on engine rotation speed. In contrast, as shown in FIG. 24B, if the horizontal axis represents engine crank angle, pressure rises become slower in accordance with the engine rotation speed becoming higher. Therefore, particularly with high loading, specifying longer injection periods in accordance with higher engine rotation speeds on a crank angle basis is unavoidable. Such injection periods becoming too long is a factor hindering increases in power output, and is not preferable.
As one technique for avoiding this, increasing fuel pressure of the pressure accumulator (common rail) in accordance with high engine rotation speeds, increasing a force which acts at the pressure intensifier, and increasing a rate of rise of fuel pressure downstream of the pressure intensification piston is available. However, in medium and high load regions, it is necessary for an injection pressure of a main injection to be a high pressure. Moreover, at this time, with a view to noise reduction and exhaust improvement, a pilot injection (injecting fuel before the main injection) or a multiple injection (a plurality of cycles of fuel injection) is implemented. However, an optimum value of injection pressure of this pilot injection is different from the main injection pressure, and is ordinarily a lower pressure than the same. A reason for this is because air temperature and density in the cylinder are low because the injection is considerably early relative to a compression dead point, and thus, if the injection pressure is set too high, penetrative force of the injection becomes excessively large and fuel adhesion at a cylinder liner surface is caused. However, in the proposed fuel injection device described above, in order to generate a high injection pressure in a high engine rotation speed region, it is necessary to raise an injection pressure that is effected at the large-bore piston of the pressure intensifier (the fuel pressure of the pressure accumulator). Therefore, an injection pressure at the time of a pilot injection, which injects fuel of the pressure accumulator just as it is, is too high compared to an optimum value, fuel adhesion to the cylinder liner surface cannot be avoided, and this is expected to be a cause for the generation of uncombusted hydrocarbons or smoke.
On the other hand, if specifications are done such that a pilot injection (fuel pressure of the pressure accumulator) and a pressure downstream of the pressure intensification piston during operation of the pressure intensifier that are suited to a time of high engine rotation speed are provided (for example, a fuel line to the large bore side of the pressure intensification piston is enlarged), a rise in the fuel pressure downstream of the pressure intensification piston during operation of the pressure intensifier at a time of low engine rotation speed is, on a crank angle basis, precipitous. Therefore, an initial period injection rate becomes too high, a pre-mixing combustion ratio increases, and NOx and noise become worse. If, in order to avoid this, fuel pressure of the pressure accumulator at times of low engine rotation speed is lowered and the initial period injection rate of the main injection is made appropriate, an atomization state of the pilot injection which injects at the fuel pressure of the pressure accumulator deteriorates, which leads to the generation of smoke.
In contrast, if, as shown in FIG. 25, the rate of rise of the fuel pressure downstream of the pressure intensification piston during operation of the pressure intensifier is set to a characteristic which increases with time, in a state in which an optimum fuel pressure of the pilot injection (fuel pressure of the pressure accumulator) is set even at high engine rotation speeds and times of high loading, the main injection can also maintain a high fuel pressure (the fuel pressure downstream of the pressure intensification piston). As a result, the problem described above can be solved, and thus it is possible to realize a low NOx, low noise, high power output engine. However, such a specification has not been possible hitherto.
Additionally, a fuel injection device equipped with a pressure intensification device has been proposed (DE 19939428 A1). However, this fuel injection device has practical objectives of improvement of injection pressure setting accuracy, durability of a nozzle seat portion, improvement of reliability and the like.
In consideration of the circumstances described above, the present invention has an object of providing a fuel injection device capable of injecting fuel by an injection pressure which is high in comparison to convention, and capable of enlarging a degree of freedom of fuel injection patterns without maximum injection pressure being determined primarily by fuel pressure of a pressure accumulator.